A known method of material treatment involves heating a particular material to a desired temperature at the location where treatment is desired. Examples of such heat treatment include soldering metallic parts, curing epoxy resins, removing plastic coatings from metals, and boring holes in solid materials. A known method of heat treatment involves focusing a laser beam on the material at the location where treatment is desired.
A particularly advantageous type of laser material processing involves the use of one or more harmonic generators to convert the energy of a laser beam having a fundamental wavelength of .lambda. into the energy of beams having harmonic wavelengths such as .lambda./2, .lambda./3, .lambda./4 and shorter wavelengths. The beams of different wavelengths are selectively used to treat materials requiring a particular wavelength beam for adequate treatment. Typically, harmonic crystals are used as the harmonic generators.
Another advantageous type of laser material processing involves the use of a diffractive optical element to convert single laser beams into a plurality of fan-out beams. The plurality of beams are then used, for example, to treat a plurality of locations on the material being treated.
Diffractive optical elements are used in many applications, such as optical connectors for computing hardware or for shaping laser beams. A body of literature exists discussing the use of diffractive optical elements to generate fan-out beams. Such references include M. P. Dames, R. J. Dowling, P. McKee, and D. Wood, "Efficient Optical Elements to Generate Intensity Weighted Spot Arrays: Design and Fabrication," Appl. Opt. 30, 2685-2691 (1991) (describing a simulated algorithm to generate a diffractive optic mark design); and K. Rastani, A. Marrakchi, S. F. Habiby, W. M. Hubbard, H. Gilchrist, and R. E. Nahory, "Binary Phase Fresnel Lenses for Generation of Two-dimensional Beam Arrays," Appl. Opt. 30, 1347-1354 (1991) (describing a technique using e-beam lithography and plasma etching to build diffractive elements in glass).
In all such references of which applicants are aware, however, the diffractive optical elements are used external to the laser unit or resonator. This means that when a laser apparatus is used that incorporates both a diffractive optical element and a harmonic generator discussed above, such as a harmonic crystal, the diffractive optical element is used to diffract the laser beam only after the harmonic crystal converts the beam to one of a harmonic wavelength.
The disadvantage of such systems is that the laser beam at the fundamental wavelength is focussed on the harmonic crystal before being diffracted by the diffractive optical element. This damages the harmonic crystal because the high intensity of the fundamental wavelength beam often causes thermal stress on the crystal and coating in these single beam harmonic generation systems. In addition, in order to generate a plurality of fan-out beams at a plurality of wavelengths in such systems, a plurality of diffractive optical elements are required. A separate diffractive optical element must be used for light beams at each wavelength. This increases the cost of the system.